The deal allows Verizon to buy blocks of radio spectrum from cable companies including Comcast, Time Warner and Cox. The companies also agreed to cross sell each other's services.
But Cardin and Mikulski worried that the arrangements will discourage Verizon from expanding its own cable business, FiOS.
"People of color and lower-income households in urban and rural parts of Maryland will be disproportionately affected by the decreased incentives to invest in continued 'build-out' of the FiOS network," the senators wrote.
They also wrote that discouraging the build-out of FiOS would prevent Verizon from hiring more workers.
Verizon argues that the deal will allow it to provide better service to its customers, and the cross-selling arrangements will provide consumers more options. The company says it never had plans to build FiOS out beyond its current footprint.
The Communications Workers of America, Verizon's union, publicized the letters. The senators noted that they had spoken with CWA members and officials who are concerned about the deal.
CWA and Verizon are in the midst of a contentious contract negotiation.
A Verizon spokesman declined to comment on the letters.